The present invention relates to a hydraulic brake system with slip control for automotive vehicles comprising a master cylinder pressurizable by means of a hydraulic power booster. Valve means are provided between the master cylinder and the wheel brakes connected to the master cylinder. Through the intermediary of the valve means a pressure medium can be drawn from the wheel brakes. The pressure medium drawn from the wheel brakes is replenishable from the pressure chamber of the hydraulic power booster, wherein a limitation of the lift of the brake pedal will occur during the slip control.
A hydraulic brake system with the characteristics described is known from the German printed and published patent application No. 30 40 562.2. The known system is equipped with a hydraulic brake power booster, wherein a booster pressure which is proportional to the actuating force is adjustable by way of a pedal-actuatable brake valve. Through the intermediary of the hydraulic power booster, a tandem master cylinder whose working chambers are connected to wheel brakes is actuatable. The connections between the working chambers of the tandem master cylinder and the wheel brakes connected thereto are interruptable by means of electro-magnetically actuatable valve means. In addition, further valve means are associated with the wheel brakes, through the intermediary of which valve means a pressure medium can be drawn from the wheel brakes, if necessary, so that the effective brake pressure will decrease which, in the event of an imminent lock-up tendency, normally has the effect that the respective vehicle wheel is sufficiently re-accelerated.
The pressure medium drawn from the wheel brakes during the brake slip control is then replenished from the pressure chamber of the hydraulic power booster, the pressure medium prevailing in the pressure chamber of the hydraulic power booster being transmitted through the intermediary of additional valve means by way of the cups of the master cylinder pistons to the corresponding working chambers. In the known brake system it is necessary, with regard to a safeguarding of a minimum volume in the tandem master cylinder, that a limitation of the lift of the brake pedal takes place. When a pressurization of the working chambers of the tandem master cylinder occurs, a so-called positioning sleeve is, for that purpose, simultaneously pressurized in opposition to a spring force in the direction of brake release. The positioning sleeve thereby moves into abutment with a stop of the booster piston and prevents a further protrusion of the booster piston and the master cylinder pistons due to the prevailing effective surface ratios.
In another brake system of the type described (patent application No. P 35 08 709.9), the booster piston penetrates into an annular chamber arranged between the master cylinder and the brake power booster, which annular chamber is connected with the pressure chamber of a "cylinder with filling steps" and with the supply reservoir, while a valve actuatable by means of the pressure in the booster chamber is interposed in the pressure medium path leading from the annular chamber to the supply reservoir. The valve closes in one switching position the fluid medium passage to the supply reservoir, thus allowing the pressure medium to flow in from the annular chamber into the pressure chamber of the "cylinder with filling steps" during the braking operation, and connects in the other switching position the annular chamber with the supply reservoir. This known brake system is disadvantageous in that a "cylinder with filling steps" is provided outside the brake actuating unit, whereby the installation space required for this brake system is increased considerably.
The present invention has as an object to create a hydraulic brake system of the type described in a fashion such that, with the system working properly, a controlled reset of the brake pedal is carried out during the control action, and that an additional pressure medium volume is simultaneously delivered into the brake circuit, while a supply of additional volume is, however, not to take place in the event of a failure of the booster in the sense of an abrupt change in ratio. In addition, the brake system is to have a simple and compact design; in particular, the valves or cylinders are to be arranged outside the housing of the unit.